Approximate Analysis of pproximate Analysis of Statically - - PDF document

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Approximate Analysis of pproximate Analysis of Statically Indeterminate Statically Indeterminate Structures Structures

Every successful structure must be capable of reaching stable equilibrium under its applied loads, regardless of structural

• behavior. Exact analysis of

indeterminate structures involves computation of deflections and solution of simultaneous

• equations. Thus, computer

programs are typically used.

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To eliminate the difficulties associated with exact analysis, preliminary designs of indeter- minate structures are often based

• n the results of approximate

analysis. Approximate analysis is based

• n introducing deformation

and/or force distribution assumptions into a statically indeterminate structure, equal in number to degree of indeter- minacy, which maintains stable equilibrium of the structure.

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No assumptions inconsistent with stable equilibrium are admissible in any approximate analysis. Uses of approximate analysis include: (1) planning phase of projects, when several alternative designs

• f the structure are usually

evaluated for relative economy; (2) estimating the various member sizes needed to initiate an exact analysis;

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(3) check on exact analysis results; (4) upgrades for older structure designs initially based on approximate analysis; and (5) provide the engineer with a sense of how the forces distribute through the structure.

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In order to determine the reac- tions and internal forces for indeterminate structures using approximate equilibrium me- thods, the equilibrium equations must be supplemented by enough equations of conditions

• r assumptions such that the

resulting structure is stable and statically determinate.

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The required number of such additional equations equals the degree of static indeter- minacy for the structure, with each assumption providing an independent relationship between the unknown reactions and/or internal forces. In approximate analysis, these additional equations are based

• n engineering judgment of

appropriate simplifying assump- tions on the response of the structure.

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Approximate Analysis of a Continuous Beam for Gravity Loads

Continuous beams and girders

• ccur commonly in building floor

systems and bridges. In the approximate analysis of con- tinuous beams, points of inflection or inflection point (IP) positions are assumed equal in number to the degree

• f static indeterminacy.

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For continuous beam struc- tures, the degree of static indeterminacy in bending (Ib) equals number of bending reactions (vertical and moment support reactions) C = number of equations

• f condition in bending

b bR

I N C 2 = − −

bR

N =

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Each inflection point position introduces one equation of condition to the static equilibri- um equations. Three strategies are used to approximate the location of the inflection points:

• 1. qualitative displacement

diagrams of the beam structure,

• 2. qualitative bending moment

diagrams (preferred method for students), and

• 3. location of exact inflection

points for some simple statically indeterminate structures.

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Approximate analysis of con- tinuous beams using the quali- tative deflection diagram is based on the fact that the elastic curve (deflected shape) of a continuous beam can generally be sketched with a fair degree of accuracy without performing an exact analysis. When the elastic curve is sketched in this manner, the actual magnitudes of deflec- tion (displacements and rota- tions) are not accurately por- trayed, but the inflection point locations are easily estimated even on a fairly rough sketch.

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Qualitative bending moment diagrams can also be used to locate inflection points. Bending moments in spans with no loading are linear or constant; with point loading the span bending moment equations are piecewise linear; and with uniform loading the moments are

• quadratic. Remember, internal

bending moments at interior support locations adjacent to one

• r two loaded spans is negative.

Recall that zero moment locations correspond to the inflection point locations.

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From the total set of inflection points, select the needed number to achieve a solution by statics. In the case of beams, there will normally be enough inflection points to reduce the structure to a statically determinate structure and typically there are more inflection points than the degree of indeterminacy.

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With the inflection points located (equal in number to the degree

• f static indeterminacy), the

analysis can proceed on the basis of statics alone. Since an inflection point is a zero moment location, it may be thought of as an internal hinge for purposes of analysis. Some examples to guide the “learning” and “practice” are given on the following pages. Both the elastic curve and bending moment diagrams are given.

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Fixed-Fixed Beam Subjected to a Uniform Load

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Fixed-Fixed Beam Subjected to a Central Point Force

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Propped Cantilever Beam Subjected to a Uniform Load

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Propped Cantilever Beam Subjected to a Central Point Force

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When considering problems that do not match the exact values given, some useful guides are:

• Inflection points move

towards positions of reduced stiffness,

• No more than one inflection

point can occur in an un- loaded span, and

• No more than two inflection

points will occur in a loaded span.

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Example Approximate Analysis

• f a Continuous Beam

L L q EI = constant IP1 IP2 L/3 βL

0.1 β < 0.25

≤

Qualitative Deflection Diagram

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Qualitative Bending Moment Diagram

• M

0.5 M L/3 βL L/3 2L/3 βL (1-β)L V1 V2

R1 M1 R2 R3

FBD through IP’s

q q

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3 2 3

q(1 )L R 2 q(1 )L V R 2 −β = −β = =

From the last FBD: From the middle FBD: 2 1 2 1

2L q( L) V LV 3 2 3q L V 4 β = + +β − β ⇒ =

2

M =

∑

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From the first FBD: y 2 1 2

F R V q L V = = + − β −

∑

2

qL R (2 5 ) 4 ⇒ = + β

2 1 1 1

L q L M M V 3 4 β = ⇒ = − =

∑

y 1 1

3q L F R V 4 β = ⇒ = = −

∑

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Shear Force Diagram

R1 R2

• R3

R2 + R1 M1

• 2M1

0.125q[(1-β)L]2 (1-β)L/2

Bending Moment Diagram

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Frame Example

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Trusses with Double Diagonals

Truss systems for roofs, bridges and building walls often contain double diagonals in each panel, which makes each panel statically indeterminate. Approximate analysis requires that the number of assump- tions introduced must equal the degree of indeterminacy so that only the equations of equi- librium are required to perform the approximate analysis.

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Since one extra diagonal exists in each double diagonal panel,

• ne assumption regarding the

force distribution between the two diagonals must be made in each panel. If the diagonals are slender, it may be assumed that the diagonal members are only capable of resisting tensile forces and that diagonals subjected to compression can be ignored since they are susceptible to buckling, i.e., assume very small buckling load and ignore post-buckling strength.

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Such an assumption is illustrated in Fig. DD.1(a). In Fig. DD.1(a), the total panel shear is assumed to be resisted by the tension diagonal as shown. Compres- sion diagonals are assumed to resist no loading. With this assumption, the truss of Fig. DD.1 is statically determinate.

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• Fig. DD.1 – Truss with Double

Diagonal Panels

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The assumption discussed in the previous paragraph is generally too stringent, i.e., the compres- sion diagonals can resist a portion of the panel shear. Figures DD.1(b) and (c) show two different assumptions regarding the ability of the compression diagonals to resist force.

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Figure DD.1(b) shows the shear (vertical) components of the diagonal members assuming that the compression and tension diagonals equally resist the panel shear. Figure DD.1(c) shows the vertical force distribution among the compression and tension diagonals based on the tension diagonal resisting twice the force

• f the compression diagonal or

two-thirds of the panel shear.

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Any reasonable assumption can be made. The compression diagonal assumptions for double diagonal trusses can be mathematically summarized as: C = αT; 0 ≤ α ≤ 1 If PS = Panel Shear, then T(1+α) = PS

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Once the diagonal member forces are determined, the remaining member forces in the truss can be calculated using simple statics, i.e., the method of sections and/or the method of joints.